NOx is a generic term for the various nitrogen oxides produced during combustion or NOx being present in off-gasses in general. Nitrogen oxides are believed to aggravate asthmatic conditions, react with the oxygen in the air to produce ozone, which is also an irritant, and eventually form nitric acid when dissolved in water. In atmospheric chemistry the term NOx means the total concentration of NO, NO2, N2O, N2O3, N2O4 and N2O5. The man made emission of NOx is of environmental concern since NOx participates in detrimental photochemical reactions in both the troposphere and the stratosphere. NOx reacts with hydrocarbon containing pollutants forming health-threatening smog in densely populated areas as well as being active in depleting the ozone layer. The end product, NO2, contributes to acid rain which can damage both trees and entire forest ecosystems. Consequently, the sources of NOx emissions are now being subjected to more stringent standards.
Nitrogen oxides can be formed during the combustion of nitrogen precursors in the fuel, defined as fuel NOx, but also from the nitrogen in the air via two mechanisms, one designated as thermal NOx, via the Zeldovich mechanism:O+N2→NO+N  [1]N+O2→NO+O  [2]N+OH→NO+H  [3]
The other is designated as prompt NOx, where the nitrogen in air is fixed by hydrocarbon radicals and subsequently oxidized to NOx [G. Löffler et al. Fuel, vol. 85, pp. 513-523, 2006]:N2+CH→HCN+N  [4]
Three primary sources of NOx formation in combustion processes are documented, the three processes being thermal NOx (reactions [1]-[3]), fuel NOx and prompt NOx (reaction [4]).
Thermal NOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting e.g. natural gas. Due to the high energy required to break the nitrogen triple bond—i.e. reaction scheme [1]—thermal NOx is primarily produced at high temperatures, usually above 1200° C. [H. Bosch et al. Catal. Today, vol. 46, pp. 233-532, 1988].
From a thermodynamic point of view, the reaction N2+O2→2NO is thermodynamically highly unfavored with a reaction enthalpy of ΔHº298 K=180 kJ/mol [G. Busca et al. Catal. Today, vol. 107-108, pp. 139-148, 2005]. Therefore it requires very high temperatures to proceed at a reasonable rate. The formation of the various nitrogen compounds, N2O, NO or NO2, depends on the oxygen partial pressure, due to the increasing O/N ratio.
Another source of NOx production from nitrogen containing fuels, such as certain coals and oil, is the conversion of chemically bound nitrogen in the fuel to NOx during combustion. The nitrogen bound in the fuel is released exemplified by the following reaction:4 NH3+5 O2→4NO+6 H2O  [5]where the nitrogen containing compounds, like ammonia and amines, are oxidized to NO. The reaction is thermodynamically highly favoured, with a reaction enthalpy of ΔHº298 K=−452 kJ, although less favoured than the oxidation of NH3 to N2. The amount of formed ‘fuel NOx’ primarily depends on the amount of nitrogen in the fuel, and is also strongly influenced by the reactor design. In natural gas (methane), nitrogen compounds are virtually absent, but substantial amounts of nitrogen is present in the case of coal, gas oils and fuel oils and especially in biofuels, such as wood.
Prompt NOx is generated when the fuel-to-air ratio is high and nitrogen radicals formed in reaction [4] react with oxygen via reaction [2]. The reactions are almost not temperature dependent, but the prompt NOx formed is negligible relative to thermal NOx.
The numerous possibilities to reduce NOx can be divided into three categories; precombustion, combustion modifications and post combustion.
The precombustion strategy imply using alternative fuels with a lower content of nitrogen species. During combustion different types of modifications can be utilized, of which the most used are; low NOx-burners, reburning (exhaust gas recirculation) and staged air combustion (thermal oxidation). A variety of other methods are also possible in the combustion modification; burners out-of-service, derating, burner system modification, trim and diluent injection.
Several post-combustion approaches are applied to reduce NOx; selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), absorption, NOx recycle, direct decomposition, photocatalytic oxidation, multifunctional filter (removal of fly-ash and NOx) and pulse intense electron beam irradiation.
A different concept is presented by wet scrubbing systems for removal of SO2, SO3 and NOx. Some aqueous scrubbing systems have been developed for the simultaneous removal of NOx and SO2 [C.-L. Yang et al. Environmental Progress, 17, 80-85 (1998)]. The wet flue gas desulfurization (FGD) typically exhibits high SO2 removal efficiencies, but the FGD can only remove a small amount of NOx because about 90-95% in a typical flue gas is present as insoluble NO and only the remaining 5-10% NO2 is water soluble.
Attempts to oxidize NO to water soluble NO2 have been made by adding strong oxidizing additives, such as MnO4− salts and H2O2, but the treatment cost involved herein has been too high for practical utilization.
Promising results of the simultaneous NO and SO2 removal in a [Co(NH3)6]2+ solution, which operates below 80° C., have been reported by Long et al. [X.-I. Long et al., Industrial & Engineering Chemistry Research, 43, 4048-4053 (2004)].
Another approach for removing NO is the complexation of NO with Fe2+-chelates based on ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetate (NTA), as outlined in reaction [6] for the case of iron-EDTA complex [F. Roncaroli et al., Coordination Chemistry Reviews, 251, 1903-1930 (2007)].Fe∥(EDTA)+NO⇄Fe∥(EDTA)(NO)  [6]
The metal-chelate can be electrochemically regenerated after absorption or reduced by sulfite ions to sulfate and nitrogen [F. Gambardella et al., Industrial & Engineering Chemistry Research 44, 4234-4242 (2005)].
In U.S. Pat. No. 6,235,248 a biotechnological approach to regenerate the iron-complex, the so-called BioDeNOx process was described. In this process the NO-saturated iron-chelate solution is brought in close contact with bacteria that regenerate the iron-EDTA complex and convert the bound nitrosyl to N2. The Fe∥(EDTA) solution needs to be somewhat diluted (concentration <200 mM) due to the presence of microorganisms, which naturally limits the absorption capacity.
The above proposed technologies for NO removal are all associated with various challenges such as low capacity, large installation footprint, poor reaction kinetics, hazardous stoichiometric reductants or oxidants, elevated reaction temperatures and the requirement for specialized catalysts.
Many of the above proposed technologies are based on liquids with a vapour pressure, which means that the solvent to some extent vaporizes during operation.
One promising solution to this particular problem could be the use of solvents referred to as ionic liquids (ILs). The expression ‘ionic liquid’ in principle encompasses any liquid entirely composed of ions (e.g. molten salts). However, within the context of this work the term will only be used to describe materials which are liquid in their pure state at room temperature. This class of solvents is often considered as ‘green’ solvents because of their immeasurably low vapour pressure. This feature gives the ILs an essential advantage over traditional solvents used for absorbing gases. Ionic liquids have already demonstrated promising behaviour in a number of reactions where gaseous reactants enter the IL solution (such as hydrogenation, hydroformylation, and oxidations) despite low gas solubilities of the gases in the IL at ambient conditions [J. L. Anthony et al. The Journal of Physical Chemistry B, 106. 7315-7320 (2002)].
Another known application of ILs is to utilize them to separate gas mixtures. WO 2007/101397 discloses gas purification processes and mentions a broad range of ionic liquids as possible absorbers of many different gasses, but does not provide any experimental evidence supporting these propositions. WO 2007/101397 is instead merely a theoretical review since there is no data evidencing how the ionic liquids work.
A promising solid ionic cation (1,1,3,3-tetramethylguanidinium) has been identified for the absorption of SO2 [J. Huang et al., Journal of Molecular Catalysis A: Chemical, 279, 170-176 (2008)]. Similarly, the solubilities of a number of gases (such as CO2, CO, O2) in imidazolium-based ILs has been reported by Anthony et al. [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002)].
Ionic liquids tend to be more viscous compared to conventional solvents, which can result in challenges regarding the mass transfer of gas into the IL. In the case of low-soluble gases, the mass transfer into the IL will likely be a rate limiting step, which can be minimized by increasing the interfacial gas-IL area and/or use high pressure systems [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002)].
Only limited information regarding the gas solubilities in ILs has been reported. Besides the reports regarding CO2 capture, the focus of most work revolves around the reactions taking place in the IL with the gas already absorbed. Only few reports exist on gas solubilities [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002); J. L. Anderson et al., Accounts of Chemical Research, 40, 1208-1216 (2007)]. The Brennecke group has, e.g. contributed with a number of seminal studies on absorption of a number of gases in imidazolium-based ILs [J. L. Anthony et al., The Journal of Physical Chemistry B, 106, 7315-7320 (2002); J. L. Anderson et al., Accounts of Chemical Research, 40, 1208-1216 (2007); J. L. Anthony et al., The Journal of Physical Chemistry B, 105, 10942-10949 (2001); J. L. Anthony et al., The Journal of Physical Chemistry B, 109, 6366-6374 (2005)].
Consequently, there is still a need for developing efficient and improved processes for removing NOx, and specifically the most abundant NOx component NO, from flue gasses from not only large stationary sources like power or incineration plants, but also from mobile emission sources like, e.g. commercial marine vessels which require a small installation footprint and low energy consumption.